SP-143 Concrete is a truly unique material, exhibiting a wide range of mechanical, physical, and chemical properties, which in turn, are affected by the type of load condition, the constituents, the local environment, the processing method, the structural application, etc. Because of this complex behavior, it is crucial that accurate and meaningful experimental methods be developed and used, in order to efficiently utilize concrete, to guarantee the public's safety, and to minimize cost. This is particularly true in the 1990s, as new and novel concretes, admixtures, and reinforcements are developed.

Various researchers have attempted to establish correlation between mechanical properties of brittle materials and ultrasonic measurements. Recently, extensive experiments, including ultrasonic scanning tests, strain gage tests, and combined ultrasonic scanning and strain gage tests, have been processed to study the degradation mechanisms and surface effects in concrete-like brittle materials. In this paper, attention is restricted to the variation of energy dissipation with external load level and the relationship between mechanically dissipated energy and ultrasonically dissipated energy for brittle materials under uniaxial compression. Some typical ultrasonic scanning readings are presented. The load-level-dependent relationship between ultrasonically dissipated energy and mechanically dissipated energy is identified and discussed. It is also pointed out that an energy-based degradation instability theory is verified qualitatively by the energy diagram obtained through the experiments. The findings may be applicable to concrete with minor modifications. However, further work would be necessary to draw a firm conclusion.

Fracture surfaces of concrete and mortar are irregular, tortuous, and stochastic in nature. To describe irregular and rough surfaces, quantitative fractographic parameters such as profile and surface roughness, fractal dimension, Fourier spectral analysis, etc., are often used. A fractal description of fracture surfaces of concrete and mortar by utilizing a new nondestructive technique, introduced by the authors, will be presented in this paper. Compact tension-fractured concrete specimens with a compressive strength of 46.8 MPa and a maximum aggregate size of 37.5 mm and a projected fracture area (ligament area) of 46,000 mm 2 (367.5 mm long by 125 mm wide), are analyzed. Through this technique, a microphotograph is taken and stored as a binary image using an image analyzer equipped with a stereo-microscope. The result is a topographical map of the fracture surface. Since the elevation of each point on the fracture surface is defined by its intensity value, the need for actual sectioning through the fracture surface, often employed, is eliminated. One-dimensional Fourier spectral analysis (1D FFT) to estimate the fractal dimension is carried out. To check the method of analysis, synthetic profiles with a known fractal dimension are generated. The results of the analysis suggest that concrete fracture surfaces are fractal for the range of scales considered, the digitized fracture surface images are found to mimic the actual fracture surfaces, their spectra follow a power lower behavior, and the technique is very promising and suitable for such materials.

The design life of bridge structures is typically 50 years. As highway authorities increasingly consider using fiber reinforced plastics (FRP) to replace steel in reinforced or prestressed concrete structures exposed to aggressive environments, it becomes imperative to develop accelerated test procedures for assessing long-term performance. While acceleration principles for determining long-term material properties, e.g., creep rupture or relaxation, are well known, no similar principles have yet been formulated for determining properties that relate to the interaction between FRP and concrete, such as bond. This is of vital importance since material durability alone cannot guarantee satisfactory performance in concrete. Paper presents a rationale for conducting accelerated tests to evaluate the long-term bond and durability characteristics of pretensioned FRPs used in bridge applications. The principles enunciated are based on recent research findings that have been translated into test setups currently being used to evaluate the long-term performance of pretensioned aramid fiber reinforced plastic (AFRP) and carbon fiber reinforced plastic (CFRP) elements exposed to a marine environment. Preliminary results obtained are quite encouraging and appear to confirm the validity of the approach used. The experimental study is scheduled to end in 1995.

Mortar and concrete samples were subjected to uniaxial compression to determine whether it is possible to distinguish two states of a sample: prior to the subjection to ultimate load, and subsequent to loading but prior to the appearance of visible surface cracks. Stress-strain characteristics, Young's moduli, and frequency characteristics of ultrasonic waves propagating through the samples were studied for each specimen. The qualitative analysis of frequencies and amplitudes of the peaks in resonant P-wave spectra allow the determination of undamaged specimens. The spectral analysis of continuous ultrasonic waves allows the possibility of discovering the specimen damaged by ultimate stress but visually intact.